The instant invention relates to a low pigment content polyester film, as well as to a process for making the same.
Pigments, especially white pigments, are known additives widely used to impart opacity or specific light transmission properties to films. However, there is currently no film designed for the sole purpose of containing (white) pigments, although there is a need for such films in applications where controlled haze, light transmission, opacity, etc., are desired final properties.
A film comprised of a mere layer containing pigment is not satisfactory, since the incorporation of fillers, especially pigments, although imparting valuable light transmission properties, presents the drawback of lowering the mechanical properties of the final film, compared to those of the host polymer prior to incorporation of the filler.
Films are widely known, and afford to combine various properties of different layers that are associated together. For example, films based on polyester are known.
JP-A-8290539 (to Diafoil Hoechst Co) discloses polyester films where the inner layer comprises fluorescent fine particles and the outer layer comprises white pigment. Said film is said to be adapted as a magnetic recording medium. The amount of white pigment is however very low.
Similarly, JP-A-8045067 (to Diafoil Hoechst Co) discloses polyester films where the inner layer comprises reclaimed raw material and the outer layer comprises white pigment. Said film is said to be adapted as a magnetic card on a pin ball game. The amount of white pigment is however very low.
JP-A-4110147 and JP-A-4110148 (to Diafoil Hoechst Co) disclose polyester films where one of the layers comprises a micropowder in an amount of from 0.1 to 5 wt %. The film has a highly delustered surface and is used for transfer moulding.
JP-A-6000859 (to Diafoil Hoechst Co) discloses polyester films where one of the layers comprises a micropowder in an amount of from 0.001 to 20 wt %, especially from 0.01 to 10 wt %. The film is said to be adapted as a magnetic recording medium, as well as a packing film, a matted film, etc. All examples make however use of very low amounts of the filler, typically about 1 wt %. Further, the process specified in said patent application is based (i) on extrusion-lamination; (ii) direct feeding with the result of the continuous polymerisation process and (iii) mixing of the filler or additive at the extruder level. This process does not make use of “chips” of polyester, and the feeding of the filler or additive at the extruder level makes it almost impossible to obtain a uniform dispersion of the filler or additive for high amounts, e.g. above 5 wt %.
Although films where one layer contains pigments are known from the above publications, it should be emphasized that these films have not been designed for the sole purposes of containing a white pigment. The problem faced when high amounts of pigments required for imparting high opacity are incorporated is that the mechanical properties of the final film are reduced. It is in fact well established that high opacity and high mechanical properties cannot be found in the same film. Further, it is also well established that high opacity requires high pigment content. Thus there is a need for a film that would show good mechanical properties together with good opacity properties, while minimizing the overall pigment content.